Not just terraforming. The magnetosphere would have to be restarted and as far as I know humans do not have the ability to do this yet. No magnetosphere, any atmosphere created would be lost and the surface dwellers would be at elevated risks from solar radiation. One mitigating control would be living under ground but any time I bring that up the HN crowd take giant dumps on it. The alternative is remaining in a space suit or pods for the rest of their unnatural lives.
All that aside humans are not mentally stable enough at the best of times to survive in pods. To your point about a colony on Mt. Everest all the biodome experiments on earth have failed and these test subjects were vetted to be physically and mentally fit. Just being in space for a prolonged period takes a massive toll on the health of astronauts glaucoma, muscle and bone loss, neurological damage. It takes several months just to get to mars so the test colony would be starting at a significant disadvantage. It would only take one unhinged person to put the lives of many pod dwellers at risk. All of this and more is just a recipe for a massively expensive disaster and that's just the known-knowns.
Yup. Even with the magnetosphere, and cleaning the toxic compounds from the soil, and presuming the gravity is no issue (perhaps boost with a centrifuge?), you'd need a couple of meters of regolith might make up for the lack of 10,300 kg of atmosphere per square meter between you and the radiation — but at that point, why bother with Mars?
Some point out that (1) the atmospheric loss is slow on timescales we care about, and (2) that anyone who can put the air on Mars in the first place can deal with the losses and replace it.
(1) Is falsified by global warming, where we saw the risks over a century ago, and yet are still having a culture war about if it's real
(2) Would be true in principle, if we were really organised. But anyone who can move that much air around is so much more advanced than us, that us trying to plan for that is probably harder than getting Ea-nāṣir personally to accurately guess the best way to build the transport infrastructure of the USA in 2025.
Just assume a technocracy on Mars. Doesn't have to be the US constitution or anything that guides them.
In fact, a constitution built around an agrarian earth community on a new unspoiled continent with abundant resources, is probably ill-suited to Mars which has none of those things.
When the Pilgrims emigrated to America, they could expect a certain level of habitability when they arrived. Water, soil, building materials. Air.
I often say, first try a colony on the top of Mount Everest. For like, a year. When you can do that, you're halfway to a Mars colony.